Transmission of optical signals (i.e., light) between different optical components requires efficient interface devices designed to facilitate the transmission of light without loss in power or leakage between elements. In general, it is desired to transfer light from a source waveguide on the source chip or from a source optical fiber into a receiving waveguide on the receiving chip or fiber, with low power loss or leakage. Waveguides on both the source and receiving chips typically have thin dimensions, and when the light is emitted out of either waveguide, it quickly diffracts in the vertical direction, typically as a Gaussian beam. This diffraction causes the exiting light to distribute optical power quickly in a vertical direction. Any gap between the two devices allows for increased divergence.
In order to mitigate such diffraction losses, the waveguides of the two optical devices would need to be brought into sub-micron proximity in order to eliminate any inter-device gap. Unfortunately, sub-micron gaps are not generally feasible due to a number of reasons, such as surface perturbations and non-uniformities in manufacturing tolerances, mechanical alignment accuracies, and interposing glue that necessarily fills the gap between the two chips that are to be bonded.
To overcome these deficiencies, lens-based and mode transforming adapters have been used to collect the diffracted light and focus it from one waveguide into the other. Unfortunately, these types of adapters are not easily fabricated in certain material systems, such as in indium phosphide compounds, where it is costly and cumbersome to etch and regrow materials repeatedly.